devmapper: Move file write and rename functionality in a separate function

Currently we save device metadata and have a helper function saveMetadata()
which converts data in json format as well as saves it to file. For
converting data in json format, one needs to know what is being saved.

Break this function down in two functions. One function only has file
write capability and takes in argument about byte array of json data.
Now this function does not have to know what data is being saved. It
only knows about a stream of json data is being saved to a file.

This allows me to reuse this function to save a different type of
metadata. In this case I am planning to save NextDeviceId so that
docker can use this device Id upon next restart. Otherwise docker
starts from 0 which is suboptimal.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
devmapper: Export nextDeviceId so that json.Marshal() can operate on it

I was trying to save nextDeviceId to a file but it would not work and
json.Marshal() will do nothing. Then some search showed that I need to
make first letter of struct field capital, exporting this field and
now json.Marshal() works.

This is a preparatory patch for the next one.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
devmapper: Save and restore NextDeviceId in a file

The way thin-pool right now is designed, user space is supposed to keep
track of what device ids have already been used. If user space tries to
create a new thin/snap device and device id has already been used, thin
pool retuns -EEXIST.

Upon receiving -EEXIST, current docker implementation simply tries the
NextDeviceId++ and keeps on doing this till it finds a free device id.

This approach has two issues.

- It is little suboptimal.
- If device id already exists, current kenrel implementation spits out
  a messsage on console.

[17991.140135] device-mapper: thin: Creation of new snapshot 33 of device 3 failed.

Here kenrel is trying to tell user that device id 33 has already been used.
And this shows up for every device id docker tries till it reaches a point
where device ids are not used. So if there are thousands of container and
one is trying to create a new container after fresh docker start, expect
thousands of such warnings to flood console.

This patch saves the NextDeviceId in a file in
/var/lib/docker/devmapper/metadata/deviceset-metadata and reads it back
when docker starts. This way we don't retry lots of device ids which 
have already been used. 

There might be some device ids which are free but we will get back to them
once device numbers wrap around (24bit limit on device ids).

This patch should cut down on number of kernel warnings.

Notice that I am creating a deviceset metadata file which is a global file
for this pool. So down the line if we need to save more data we should be
able to do that.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>